Since a solar cell as a photoelectric conversion device to convert sunlight into an electric energy uses the sunlight as an energy source, an influence that is exercises to the global environment is extremely small and it is expected to be spread further.
Various materials have been examined as a material of the solar cell and a number of solar cells using silicon have been commercialized. They are mainly classified into: a crystalline silicon solar cell using single crystal silicon or poly crystal silicon; and an amorphous silicon solar cell. Hitherto single crystal silicon or poly crystal silicon, that is, crystalline silicon has often been used for the solar cells.
However, in the crystalline silicon solar cell, although photoelectric conversion efficiency indicative of performance of converting the light (solar) energy into the electric energy is higher than that of the amorphous silicon solar cell, since a large energy and a long time are required for a crystal growth, productivity is low and it is disadvantageous in terms of costs.
Although higher light absorptivity, wider selecting range of a substrate, easier realization of a large area, and the like are characteristics of the amorphous silicon solar cell than in the case of the crystalline silicon solar cell, the photoelectric conversion efficiency is lower than that of the crystalline silicon solar cell. Further, in the amorphous silicon solar cell, although the productivity is higher than that of the crystalline silicon solar cell, a vacuum process is necessary upon manufacturing in a manner similar to the crystalline silicon solar cell and a facility related cost is still heavy.
In order to solve the above problems and realize the even lower costs of the solar cell, solar cells using an organic material in place of a silicon material have been studied for a long time. However, since most of those solar cells have the low photoelectric conversion efficiency of about 1%, they are not put into practical use.
Among them, according to the dye sensitized solar cell proposed by Glötzl et al. in 1991, since it is reasonable in price, shows the high photoelectric conversion efficiency and, unlike the conventional silicon solar cell, a large apparatus is unnecessary upon manufacturing, and the like, attention has been paid (for example, Nature, 353, p. 737 (1991)).
According to a general structure of such a dye sensitized solar cell, a semiconductor porous electrode in which a sensitizing dye is combined with a semiconductor porous membrane of a titanium oxide or the like formed on a transparent conductive substrate and a counter electrode obtained by forming a platinum layer or the like onto the substrate are combined, and an organic electrolytic solution containing redox species such as iodine, iodide ions, or the like is filled between both electrodes.
The semiconductor porous electrode which is used can be obtained by a method whereby semiconductor fine grain (titanium oxide fine grain or the like) and a polymer compound such as polyethylene glycol, polystyrene, or the like serving as a binder are mixed, coated onto the transparent conductive substrate by a doctor blade method, a spin coating method, a dip coating method, or the like, and thereafter, sintered at temperatures of 400 to 500° C. for 30 minutes to one hour. The semiconductor porous electrode is constructed by a semiconductor layer (or semiconductor thin film) made of semiconductor fine grain having a fine grain diameter of about 20 to 30 nm and has a structure in which many fine holes in that a diameter of tens of nm is set to the center of distribution exist in the electrode. The titanium oxide porous electrode as a semiconductor porous electrode suitable for the photoelectric conversion device among them is an anatase-type fine grain thin film in which a grain diameter is small, a specific surface area is large, and a photocatalyst activity is high.
It has been reported that if the semiconductor fine grain is made of the titanium oxide, its surface changes to the surface having hydrophilicity (surface hydroxyl group increases) by irradiating ultraviolet rays (Nature, 388, p. 431 (1999)).
However, according to the knowledge obtained uniquely by the inventors et al. of the present invention, it has been found that if a sintering temperature is lowered or a sintering time is shortened in order to suppress the crystal growth and keep the crystal grain diameter small, a large amount of organic substance derived from the polymer compound used as a binder remains in the semiconductor porous electrode. This obstructs combination of the semiconductor fine grain, resulting in deterioration of the photoelectric conversion efficiency. On the contrary, if the sintering temperature is raised or the sintering time is extended in order to reduce the residual amount of the organic substance, the crystal grain diameter increases, the specific surface area decreases, and the structure changes to the crystalline structure in which the photocatalyst activity is low (in the case of titanium oxide, rutile type). Also in this case, the photoelectric conversion efficiency deteriorates.
Therefore, the problem to be solved by the invention is to provide a photoelectric conversion device which has such a crystalline structure (for example, an anatase type in the case of titanium oxide) that an amount of residual organic substance in a semiconductor layer made of semiconductor fine grain is extremely small, a crystal grain diameter of the semiconductor layer is small, a specific surface area is large, and a photocatalyst activity is high and whose photoelectric conversion efficiency is high and to provide a manufacturing method of such a device.
More generally, the problem to be solved by the invention is to provide an electronic apparatus which has such a crystalline structure that an amount of residual organic substance in a semiconductor layer made of semiconductor fine grain is extremely small, a crystal grain diameter of the semiconductor layer is small, a specific surface area is large, and a photocatalyst activity is high and whose characteristics are excellent and to provide a manufacturing method of such an electronic apparatus.